1. Field of the Invention
The present invention is directed to a fogged direct positive emulsion, and particularly a method of nonuniform fogging to produce a long scale (extended latitude) emulsion having a smooth and continuous Density vs. Log of Exposure (Log E) curve.
2. Description of the Prior Art
The simplest route to positive-positive photographic reproduction is provided by certain types of silver halide emulsions that are fogged in manufacture, and, on exposure and conventional development, yield direct positive images of the original subject matter. These emulsions are called prefogged direct positives. To give faithful reproductions it is desirable that the Density vs. Log Exposure curves of such emulsions have an extended exposure latitude, or long scale. In addition it is desirable that the curve shape be smooth and continuous.
Smith et al, in U.S. Pat. No. 3,615,573 "Direct-Positive Composition Containing Individually and Differently Fogged Silver Halide Emulsions" addressed the problem of extending the exposure latitude of a fogged direct positive emulsion by separating an unsensitized emulsion into two or more portions, individually fogging them to different levels, and applying these to a support, either in separate layers or as a blend. The result was a Density vs. Log E curve having one or more discrete high-contrast steps of exposure range of the photographic composition.
FIG. 1 of the drawings is a reproduction of a highly preferred embodiment of Smith et al, viz. FIG. 3 of U.S. Pat. No. 3,615,573. This illustrates how three separate emulsions of different degrees of chemical fogging can be combined to provide extended exposure latitude. While illustrating the steps which are produced by the combination of emulsions, this Figure also shows that techniques which involve mixing of emulsions of different sensitivities give Density vs. Log E curves which contain breaks, i.e., there are sharp changes in direction. Prior art techniques of mixing emulsions are characterized by such breaks since the net curve is really a superpositioning of the curve shapes of the different emulsions used and the break is representative of a transition from one emulsion to another.
However, given the complex nature of photographic emulsions, it is difficult to control the fogging process for even a single emulsion, much less maintain proper control over several emulsions so that, if desired, the transition from one step to another is more or less "smooth". Also, the blending of two emulsions of different speeds to produce a step or flat spot in the Density vs. Log E curve causes a region of reduced contrast in the midtones of the duplicate. The present invention proceeds in the opposite direction and provides a simpler means to extend exposure latitude, using only a single emulsion to obtain an essentially smooth curve.